With the rapid development of information technologies, the submarine cable network covering all major seas around the world has become an important communication network that bears important international communication services. In order to guarantee smooth communication services, it makes great sense to monitor a submarine cable system. Due to unique characteristics of a submarine optical cable communication system, an online test, that is, direct online monitoring without interrupting the service, is needed sometimes. Therefore, different loop structures are employed for monitoring the submarine cable system, and special test loops are used to feed back monitoring signals.
Currently, the principle of monitoring a submarine cable system is mainly as follows. Optical pulses are emitted into an input optical fiber. Because the optical pulses are scattered by scattering particles, or reflected by fractured surfaces of the optical fiber due to Fresnel reflection, the Fresnel-reflected light and Rayleigh back-scattered light are sent into a receiver by a light beam splitter, and then are transformed into electrical signals and displayed on an oscilloscope together with the changing time. In this way, a monitoring result is obtained.
In the prior art, a Coherent Detection Optical Time Domain Reflectometer (COTDR) is used to monitor a submarine cable system with a modulation format of Amplitude Shift Keying (ASK) or Frequency Shift Keying (FSK), and then coherent reception is performed. For FSK, in essence, a signal of one frequency serves as detection light, and a signal of another frequency serves as filling light. By using the filling light, the surge phenomenon of an Erbium-doped Optical Fiber Amplifier (EDFA) is eliminated, and the performance of the COTDR is improved.
FIG. 1 is a schematic diagram of an application scenario of a COTDR in the prior art. The COTDR uses the Rayleigh scattering theory to monitor a submarine cable system. The Rayleigh scattering is a type of random scattering that occurs when an optical signal is transmitted along an optical fiber. The COTDR just monitors a part of the Rayleigh-scattered light that enters a light-receiving port of the COTDR. The location of a fault point is determined according to the power of the part of the Rayleigh-scattered light. For example, if the power drops abruptly at a time point on the curve of a monitoring result, it is deemed that a fault exists in the part of the submarine cable system corresponding to the time point, and the distance between the fault point and a land terminal can be calculated according to the time point, so that the location of the fault point is obtained. A special link needs to be designed for a signal feedback channel, and in this way, reflected light of the uplink channel and emitted optical pulse signals may enter the downlink channel through a light-emitting port of the COTDR and be reflected back.
In another solution for monitoring a submarine cable system, detection is performed by emitting light of multiple wavelengths to obtain a monitoring result. In the solution, a Distributed Bragg Reflector (DBR) laser is used to modulate frequencies in sequence with light of one wavelength emitted each time, and then the detection is performed.
After analyzing the prior art described above, the inventors find the following.
1) In a submarine cable system, when a COTDR is used to monitor the submarine cable system, in order to get a precise monitoring result, the monitoring result obtained each time needs to be averaged for many times, normally 216 times or even 224 times. Therefore, when a length of 5,000 km needs to be monitored, as each time of monitoring takes at least 0.05 s, the total time required for monitoring is about one hour (216 times) or more, and such long monitoring time may greatly affect the monitoring efficiency. Generally speaking, the monitoring time is long, and the performance of real-time monitoring is poor.
2) If a single tunable laser is applied to generate serial light of multiple wavelengths to perform detection, as coherent reception is based on the frequency of the filling light, only after the detection pulse is emitted and the frequency of the detection light changes to the frequency of the filling light can the waveform be detected. If more wavelengths are adopted, the monitoring blind zones are larger.
Because data is required to be aligned and accumulated at a receiver end, a large quantity of sampling data makes the system complex. In addition, the accumulating and buffering the large quantity of sampling data needs considerable data space and time.
The wavelength adjustment of the DBR laser takes time, which is not convenient for detection.